1. Field of the Invention
The present invention relates to battery-operated power tools having light sources for illuminating the work area.
2. Background Art
The use of battery-operated power tools has become widespread. Some of these tools are provided with a light source for illuminating the work area. One approach for providing the light source is to provide a simple switch for turning the light source on and off. Another approach is to provide a momentary contact switch in conjunction with a timer circuit so that the momentary assertion of the switch causes the light to turn on and remain on until the expiration of a predetermined time delay. Most timer circuits that use a time delay device such as a 555 integrated circuit timer require the battery voltage to be powering the device before and after the desired time delay has been activated. The power on the timer will constantly drain the battery whether the time delay circuit has been activated or not. After sufficient time, the battery will become totally discharged resulting in total battery failure.
Some background information may be found in U.S. Pat. Nos. 6,318,874, 5,473,519, 5,179,325, and 5,169,225. U.S. Pat. No. 6,318,874 describes a power tool having a lighting device. In that patent, a single switch causes the motor and the light to operate at substantially the same time and there is no way to turn on the light without actuating the motor.
For the foregoing reasons, there is a need for a battery-operated power tool with a light source that avoids the problem of constant drain on the battery, and avoids limitations associated with other existing designs.
It is, therefore, an object of the present invention to provide an improved battery-operated power tool utilizing a driver circuit for implementing time delay turn off of a light source.
In carrying out the above object, a battery-operated power tool is provided. The power tool comprises a housing, a motor disposed in the housing, a battery, a light source, a driver circuit, and first and second physical switches. The driver circuit includes a transistor configured as a switch connecting the battery to the light source. The driver circuit further includes a capacitor arranged to form a timer. The momentary charging of the capacitor causes the transistor switch to close and to remain closed for a predetermined period of time while the capacitor at least partially discharges. The first physical switch is arranged such that assertion of the first physical switch charges the capacitor. The second physical switch is separate from the first physical switch. The second physical switch is arranged such that assertion of the second physical switch connects the battery to the motor.
It is appreciated that the light source may be a light emitting diode (LED) or other suitable light source. It is appreciated that the driver circuit transistor may be a bipolar junction transistor (BJT), a field effect transistor (FET) or any other suitable transistor. It is appreciated that the capacitor may be arranged to form the timer in a variety of ways, for example, having the capacitor discharge through the transistor, through a resistor, or through both the transistor and a resistor. It is appreciated that the momentary charging and subsequent discharging of the capacitor may take place in a variety of ways depending on the transistor configuration (for example, npn BJT, pnp BJT, n-channel FET, or p-channel FET).
At a more detailed level, the invention comprehends a zener diode arranged such that the zener diode voltage drives the light source. This provides a constant drive on the light source and near constant light intensity level. In the preferred embodiment, the transistor is a bipolar junction transistor (BJT). More preferably, the light source is connected at the emitter of the bipolar junction transistor (BJT) as opposed to other possible connections such as at the collector.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the preferred embodiment when taken in connection with the accompanying drawings.